Polymeric fiberglass binders have a variety of uses ranging from stiffening applications where the binder is applied to woven or non-woven fiberglass sheet goods and cured, producing a stiffer product; thermoforming applications wherein the binder resin is applied to sheet or lofty fibrous product following which it is dried and optionally B-staged to form an intermediate but yet curable product; and to fully cured systems such as building insulation, wherein the binder is fully cured to its thermoset state while the fiberglass is in the fully expanded condition, following which the rolls or batts are compressed for storage and shipment. In the latter case, it is important that upon releasing the compression, that the batt or roll of fiberglass insulation recover a substantial part of its precompressed thickness.
Polymeric binders used in the present sense should not be confused with matrix resins which are an entirely different and non-analogous field of art. While sometimes termed "binders", matrix resins act to fill the entire interstitial space between fibers, resulting in a dense, fiber reinforced product where the matrix must translate the fiber strength properties to the composite, whereas "binder resins" as used herein are not space-filling, but rather coat only the fibers, and particularly the junctions of fibers. Binder resins in these applications perform no translation of fiber strength. Rather, the unique physical properties of these products are related in general to polymer stiffness rather than fiber strength. Fiberglass binders also cannot be equated with paper or wood product "binders" where the adhesive properties are tailored to the chemical nature of the cellulosic substrates. Many such resins, e.g. urea/formaldehyde and resorcinol/formaldehyde resins, are not suitable for use as fiberglass binders. One skilled in the art of fiberglass binders would not look to cellulosic binders to solve any of the known problems associated with fiberglass binders.
From among the many thermosetting polymers, numerous candidates for suitable thermosetting fiberglass binder resins exist. However, binder-coated fiberglass products are often of the commodity type, and thus cost becomes a driving factor, ruling out such resins as thermosetting polyurethanes, epoxies, and others. Due to their excellent cost/performance ratio, the resins of choice in the past have been phenol/formaldehyde resins. Phenol/formaldehyde resole resins can be economically produced, and can be extended with urea prior to use as a binder in many applications. Such urea-extended phenol/formaldehyde resole binders have been the mainstay of the fiberglass insulation industry for years, for example.
Over the past several decades however, minimization of volatile organic compound emissions (VOCs) both on the part of the industry desiring to provide a cleaner environment, as well as by Federal regulation, has led to extensive investigations into not only reducing emissions from the current formaldehyde-based binders, but also into candidate replacement binders. For example, subtle changes in the ratios of phenol to formaldehyde in the preparation of the basic phenol/formaldehyde resole resins, changes in catalysts, and addition of different and multiple formaldehyde scavengers, has resulted in considerable improvement in emissions from phenol/formaldehyde binders as compared with the binders previously used. However, with increasingly stringent Federal regulations, more and more attention has been paid to alternative binder systems which are free from formaldehyde.
One such candidate binder system employs polymers of acrylic acid as a first component, and a polyol such as glycerine or a modestly oxyalkylated glycerine as a curing or "crosslinking" component. The preparation and properties of such poly(acrylic acid)-based binders, including information relative to the VOC emissions, and a comparison of binder properties versus urea formaldehyde binders is presented in "Formaldehyde-Free Crosslinking Binders For Non-Wovens", Charles T. Arkins et al., TAPPI JOURNAL, Vol. 78, No. 11, pages 161-168, November 1995. The binders disclosed by the Arkins article, appear to be B-stageable as well as being able to provide physical properties similar to those of urea/formaldehyde resins. Unfortunately, urea/formaldehyde resins do not in general offer the same properties as phenol/formaldehyde resins, the most widely used fiberglass binder resins.
U.S. Pat. No. 4,076,917 discloses .beta.-hydroxyalkylamides, more particularly bis(.beta.-hydroxyalkylamides) as curing agents for polymers containing carboxyl functionality. Numerous unsaturated monomers are disclosed for preparation of the carboxyl-functional polymer, and copolymers of ethylacrylate/methacrylic acid, and ter- and tetrapolymers of butylacrylate/methylmethacrylate/styrene/methacrylic acid; ethylacrylate/styrene/methacrylic acid; butyl acrylate/methacrylic acid/styrene/maleic anhydride; and ethylacrylate/methylmethacrylate/methacrylic acid are among the carboxylic acid group-containing polymers exemplified.
U.S. Pat. No. 5,108,798 discloses water soluble binders prepared from polyfunctional carboxylic acids and .beta.-hydroxyurethanes. Among the polycarboxylic acids, preference is given to monomeric polycarboxylic acids such as the cycloalkane tetracarboxylic acids and anhydrides, pyromellitic acid and its anhydride, and maleic acid and its anhydride. Polymaleic acid and polymaleic anhydride are also identified. Poly(acrylic acids) are exemplified as not producing cured products with good tensile strength.
U.S. Pat. No. 5,143,582 discloses heat resistant non-wovens containing ammonia-neutralized polycarboxylic acids, either monomeric or polymeric, and .beta.-hydroxyalkyl amides. High molecular weight poly(acrylic acid) is shown to be superior to low molecular weight poly(acrylic acid) in these applications. Apparent cure temperature is 204.degree. C. However, the binder compositions are believed to liberate ammonia upon cure. Ammonia emissions are becoming increasingly tightly regulated.
U.S. Pat. No. 5,318,990 discloses fiberglass insulation products cured with a combination of a polycarboxy polymer, a .beta.-hydroxyalkylamide, and an at least trifunctional monomeric carboxylic acid such as citric acid. No polycarboxy polymers other than poly(acrylic acid) are disclosed, although co- and terpolymer polycarboxy acids are broadly disclosed.
Published European Patent Application EP O 583 086 A1 appears to provide details of polyacrylic binders whose cure is catalyzed by a phosphorus-containing catalyst system as discussed in the Arkens article previously cited. European Published Application EP O 651 088 A1 contains a related disclosure pertaining to cellulosic substrate binders. The fiberglass binders of EP '086 are partially neutralized polycarboxy polymers and hydroxyl-functional curing agents wherein the polycarboxy polymers are prepared in the presence of sodium hypophosphite, incorporating the latter into the polymer structure or by incorporating sodium hypophosphite separately into the curable mixture of polycarboxy polymers to serve as the curing catalyst. Terpolymers of acrylic acid, maleic acid, and sodium hypophosphite are exemplified but appear to reduce both dry and wet tensile strength as compared to poly(acrylic acid) catalyzed with sodium hypophosphite. Higher molecular weight poly(acrylic acids) are stated to provide polymers exhibiting more complete cure. Under the same conditions, copolymers of acrylic acid and maleic acid are shown to have less complete cure as shown by the swell ratios of the polymers, and the copolymer with higher maleic acid content fared worse in this comparison.
Further, and most importantly, as Arkens indicates, the normal cure temperature of the acrylic binder resins is approximately 180.degree. C., and a final cure does not take place without prolonged heating at this temperature or by allowing the temperature to rise to the range of 220.degree. C. to 240.degree. C. The combination of curing temperature and cure time necessitates thermal energy requirements considerably in excess of what is normally desirable for phenol/formaldehyde resins. While it might seem that a simple increase in furnace temperature could provide the additional thermal energy required, it must be remembered that in a commercial setting, the exceptionally large furnaces, powerful heat supplies, and ancillary equipment must all be changed if a binder with higher thermal energy curing requirements is to be used. These changes are not minimal, and represent a considerable financial investment, in many cases requiring significant additional furnace length. Moreover, it is highly likely that for a considerable period of time at least, a variety of binder resins may be used on the same line at different times. Thus, any change made to the curing ovens must be easily reversible. Thus, poly(acrylic acid) binder systems having curing energy requirements similar to those of phenol/formaldehyde binders would be desirable.
The cure temperatures and rates of cure are believed to be dependent upon a number of factors. These, of course, include the reactivity of the carboxylic acid and polyol and the presence and type of esterification catalyst present. The poly(acrylic acid) polymer and the polyol together contain far more theoretical crosslinking possibilities than is believed necessary to provide the necessary crosslinking to attain a thermoset binder. It is believed that a significant fraction of carboxylic acid groups from the poly(acrylic acid) and hydroxyl groups from the polyol in fact do not esterify, but remain unreacted in the thermoset product. One of the reasons for the difficulty of esterification of poly(acrylic acid) carboxylic acid groups and polyol hydroxyl groups is that poly(acrylic acid) is believed to form self-associating discrete phases upon loss of water solvent, possibly to the exclusion of the polyol present. Crosslinking via esterification can then only occur when sufficient thermal energy is present to disrupt these discrete phases. Such molecular disruption may occur solely via thermally-induced molecular motions, and/or by a change in the poly(acrylic acid) molecule caused by internal anhydride formation between neighboring carboxylic acid groups.